"These THUGS are dishonouring the memory of George Floyd, and I won’t let that happen. Just spoke to Governor Tim Walz and told him that the military is with him all the way. Any difficulty and we will assume control but, when the looting starts, the shooting starts. Thank you!"

The Minneapolis police station for the third precinct - which housed the officers connected to Floyd's death - was set on fire by protesters who had also looted local businesses, according to the twitter account of the Star Tribune.

Another Star Tribune journalist, Mark Vancleave, tweeted that a string of gunshots had been heard near the precinct, it's unclear who was firing the shots.

It was the third consecutive night of violent protests following Floyd's death on Monday. In footage recorded by a bystander, Floyd, a black man, can be seen pleading that he can't breathe as Officer Derek Chauvin, who is white, kneels on his neck. As minutes pass, Floyd slowly stops talking and moving.

The prospect of deliberate disinformation worsening the situation prompted the Minneapolis police department to identify a false rumour claiming that one of its police officers was encouraging lawless activity.

"We are aware of the social media post that erroneously identifies one of our officers as the person caught on video breaking windows in Minneapolis," the tweet said. "We've seen it. We've looked into it. And it's false."

Dozens of businesses across the Twin Cities were boarding up their windows and doors in an effort to prevent looting, with the Minneapolis-based Target announcing it was temporarily closing two dozen stores. Minneapolis shut down nearly its entire light-rail system and all bus service until Sunday out of safety concerns.

In St Paul, clouds of smoke hung in the air as police armed with batons and wearing gas masks and body armour kept a watchful eye on protesters along one of the city's main commercial corridors, where firefighters also sprayed water onto a series of small fires.

At one point, officers stood in line in front of a Target store, trying to keep out looters, who were also smashing windows of other businesses.

Hundreds of demonstrators returned to the Minneapolis neighbourhood at the centre of the violence, where the night-time scene oscillated between an angry protest and a street party.

Earlier, a band playing in a parking lot across from the police's third precinct station, the focus of the protests, broke into a punk version of Bob Marley's Redemption Song. Nearby, demonstrators carried clothing mannequins from a looted store and threw them onto a burning car. Later, a building fire erupted nearby.

As night descended, people swung baseball bats and fireworks abruptly lit up the sky. Some people carried large boxes from shops as police helicopters lingered overhead.

But elsewhere in Minneapolis, thousands of peaceful demonstrators marched through the streets calling for justice.

Earlier on Thursday (Friday AEST), Minnesota Governor Tim Walz called in the National Guard to try to stem the violence. The Guard said 500 members had been mobilised.

Floyd's death has deeply shaken Minneapolis and sparked protests in cities across the US. Local leaders have repeatedly urged demonstrators to avoid violence.

"Please stay home. Please do not come here to protest. Please keep the focus on George Floyd, on advancing our movement and on preventing this from ever happening again," tweeted St Paul Mayor Melvin Carter, who is black.

Erika Atson, 20, was among thousands of people who gathered outside government offices in downtown Minneapolis, where organisers had called a peaceful protest. Many protesters wore masks because of the coronavirus pandemic, but there were few attempts at social distancing.

Atson, who is black, described seeing her 14 and 11-year-old brothers tackled by Minneapolis police years ago because officers mistakenly presumed the boys had guns. She said she had been at "every single protest" since Floyd's death and worried about raising children who could be vulnerable in police encounters.

"We don't want to be here fighting against anyone. We don't want anyone to be hurt. We don't want to cause any damages," she said. "We just want the police officer to be held accountable."

Hennepin County Sheriff David Hutchinson said the rally had been peaceful and there had been no arrests by late evening.

After calling in the guard, the governor urged widespread changes in the wake of Floyd's death.

"It is time to rebuild. Rebuild the city, rebuild our justice system and rebuild the relationship between law enforcement and those they're charged to protect," Walz said.

Much of the Minneapolis violence occurred in the Longfellow neighbourhood, where protesters converged on the precinct station. In a strip mall across the street, the windows of nearly every business had been smashed, from the large Target department store at one end to the Planet Fitness gym at the other. Only the 24-hour laundromat appeared to have escaped unscathed.

"WHY US?" demanded a large expanse of red graffiti scrawled on the wall of the Target store. A Wendy's restaurant across the street was charred almost beyond recognition.

Among the casualties of the overnight fires: a six-storey building under construction that was to provide nearly 200 apartments of affordable housing.

"We're burning our own neighbourhood," said a distraught Deona Brown, a 24-year-old woman standing with a friend outside the precinct station, where a small group of protesters were shouting at a dozen or so stone-faced police officers in riot gear. "This is where we live, where we shop, and they destroyed it." No officers could be seen beyond the station.

"What that cop did was wrong, but I'm scared now," Brown said.

Others in the crowd saw something different in the wreckage.

Protesters destroyed property "because the system is broken", said a young man who identified himself only by his nickname, Cash, and who said he had been in the streets during the violence. He dismissed the idea that the destruction would hurt residents of the largely black neighbourhood.

"They're making money off of us," he said angrily of the owners of the destroyed stores. He laughed when asked if he had joined in the looting or violence. "I didn't break anything."

The protests were more violent than Tuesday's, which included skirmishes between offices and protesters but no widespread property damage.

Mayor Jacob Frey appealed for calm but the city's response was quickly questioned as things started spiralling into violence. "If the strategy was to keep residents safe - it failed," City Council Member Jeremiah Ellison, who is black, tweeted. "Prevent property damage - it failed." He urged police to leave the scene of violence, saying their presence brought people into the streets.

But Eric Kowalczyk, a police captain in Baltimore during the Freddie Gray riots in 2015, generally supported the Minneapolis police strategy to avoid confrontations when possible, saying heavy-handed police responses were only met with more violence.

"Nobody wants to see their city on fire, but at the same time, you don't want to see citizens injured by the very police department they are protesting," he said.

Protests have also spread to other cities. In New York City, protesters defied coronavirus prohibition on public gatherings, clashing with police, while demonstrators blocked traffic in downtown Denver, Colorado. A day earlier, demonstrators had taken to the streets in Los Angeles and Memphis.

Amid the violence in Minneapolis, a man was found fatally shot on Wednesday night near a pawn shop, possibly by the owner, authorities said.

Fire crews responded to about 30 intentionally set blazes, and multiple fire trucks were damaged by rocks and other projectiles, the fire department said. No one was hurt by the blazes.

Also on Thursday, the city released a transcript of the 911 call that brought police to the grocery store where Floyd was arrested. The caller described someone paying with a counterfeit bill, with workers rushing outside to find the man sitting on a van. The caller described the man as "awfully drunk and he's not in control of himself.". Asked by the 911 operator whether the man was "under the influence of something," the caller said: "Something like that, yes. He is not acting right." Police said Floyd matched the caller's description of the suspect.

The US Attorney's Office and the FBI in Minneapolis said they were conducting "a robust criminal investigation" into the death. President Donald Trump has said he had asked an investigation to be expedited.

The FBI was also investigating whether Floyd's civil rights were violated.

Chauvin, the officer who kneeled on Floyd's neck, was fired on Tuesday along with three other officers involved in the arrest. The next day, the mayor called for Chauvin to be criminally charged.

AP, staff reporters